1. Field of the Invention
The present invention generally relates to apparatus and methods of controlled bending of a pipeline beyond elastic limits during the laying of the pipeline in the sea. In particular, the present invention relates to apparatus and methods that utilize external forces to produce controlled bending of a pipeline beyond elastic limits. The present invention also relates to bending sleeves that are used to limiting the amount of bending that can occur in the pipeline during the laying of the pipeline or during the maintaining of the pipeline at the sea floor.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Subsea pipelines are typically fabricated one segment at a time aboard a pipeline-laying vessel. As each segment is added, the vessel moves forward and the pipeline follows a descending path to the sea floor. The suspended pipe span between the vessel stern and the sea floor is typically supported partially by a ramp attached to the vessel stern and axial tension is applied to the pipe which maintains the pipeline within elastic boundaries.
The steel pipelines that are laid on the bottom of the sea cannot be pre-formed or pre-adapted to the contour of the sea floor. This is because of the above-identified laying and installation procedure.
There may be tolerated a certain degree of unevenness over which the pipeline is capable of spanning or bending, provided that the specific load does not produce excessively high stresses in the pipeline steel or cause vortex-induced vibrations. If stresses exceed allowable limits, the pipeline could be deformed permanently, either by buckling or cold bending, or both to an unacceptable configuration. Should cold bending occur through yielding of the steel in the pipeline, it could propagate uncontrollably. Requirements set by classification societies for construction and operation of offshore pipelines permit a certain degree of cold bending beyond elastic limits provided that it takes place under controlled conditions. Parameters for such controlled conditions entail that a pipeline may be cold bent to a minimum radius less than what is allowed for uncontrolled bending.
Pipelines that are laid on an uneven sea floor are subjected to free spanning because of the rigidity of the pipeline. Specifications used for submarine pipeline installation permits plastic deformation as long as positive measures are taken to ensure that excessive bending is prevented. By allowing plastic deformation, it is possible to reduce to a considerable degree the occurrence of free spanning. Bending beyond elastic limits may be achieved by overloading the pipeline by applying external loads.
Submarine pipelines having a diameter of more then twelve inches usually require a weight coating to achieve negative buoyancy. This is necessary if the pipeline is to be submerged and also maintain a stable state with respect to the sea current. Plastic deformation of a pipeline having a weight coating of concrete will cause the concrete to crack and break loose.
In the past, various patents and publications have issued relating to the controlling of the bending of a pipeline. For example, U.S. Pat. No. 5,192,166, issued to the present inventor, describes a method for controlled bending of a pipeline during the laying thereof in the sea. This method utilizes bend controlling/stopping means which are mounted on the pipeline and interact with the pipeline. To achieve cold bending under controlled conditions, the pipeline is weight-loaded internally at the selected bending zone. The weight-loading may be achieved by means of a flexible string of weight elements and/or by introducing into the pipe a suitable heavy, readily flowable weight mass, for example, drilling fluid or water.
U.S. Pat. No. 5,403,121, issued on Apr. 4, 1995 to Lanan, describes a method for accommodating thermal expansion of a buried subsea pipeline. This method includes the steps of providing a pipeline which bends in alternating essentially opposed directions. The angles of the bends and the distance between the bends are sufficiently small so that the pipeline is not plastically deformed when one end of the pipeline is suspended from the surface of the sea. The number and angles of the bends are sufficiently large to prevent upheaval buckling. The bend angles and distance between the bends is small enough that the pipeline can be passed through a tensioning machine and stinger of a pipeline-laying vessel. The pipeline can then be installed on the sea floor.
U.S. Pat. No. 1,677,077, issued on Jul. 10, 1928 to D. D. Fortune, describes a hose protector in which a flexible sheath is connected to a sleeve. The sheath comprises a plurality of sections pivotally connected together. Each section is formed of a plurality of rings. Straps connect the rings together. An end of each strap projects beyond a ring and is pivotally connected to a ring of an adjacent section. The sleeve is internally threaded so as to form a plurality of section pivotally connected together.
U.S. Patent Publication No. 2010/0329792, published on Dec. 30, 2010 to Persson, describes a controlled bending of a pipeline by an external force. A bending sleeve assembly is attached to a selected bending zone of the pipeline so as to limit the control bending of the pipeline to a predetermined resulting bending configuration. External force is then applied on the bending sleeve assembly so as to bend the pipeline to the resulting bending configuration in cooperation with the bending sleeve assembly and the sea floor. The external force can be exerted from one or more weights placed on top of the bending sleeve assembly.